Why do your pupils dilate when you smoke marijuana? What is the ingredient that does this? And how?

— One-Hit Wonder

Dear One-Hit Wonder,

No two tokers are alike — that is, people may experience differential effects when using marijuana. How marijuana affects the pupil diameter is somewhat controversial, and whatever causes changes in pupil size is not well understood. Some studies reported increases in pupil size, others reported decreases, and some studies even found no changes in pupil size. Moreover, studies have found that changes in pupil size are so slight, that they may be imperceptible to the naked eye.

It is possible that a person who experiences dilated pupils after smoking marijuana had a joint that was laced with another added substance. Depending on the source of the marijuana it can be difficult to tell if it has been altered with other substances or chemicals. And as the quality and mix of marijuana varies widely, there may not be a clear answer to your questions (yet).

Speaking of marijuana impacts on the eyes, conjunctival injection, or redness of the white part of the eye, is a common physical side effect of smoking marijuana. While somewhat uncommon at low blood THC concentrations, it becomes increasingly common as blood THC concentrations increase. This is thought to be due to the vasodilation (the widening of blood vessels) of the capillaries of the eye.